This invention relates to wireless office systems and, more particularly, to an automatic frequency allocation method and system.
A wireless communication system, in the form of a cellular system, is designed to cover a large geographic area. The system is divided into numerous cells providing air interface between mobile stations and land-based systems. Each cell includes a base station for communicating with mobile stations. These wireless communication systems maintain a set of frequencies that are used for traffic channels and control channels. Frequency planning is necessary in order to determine which of the frequencies should be used at any given time. Automatic frequency allocation algorithms assign specific frequencies to each base station to provide efficient service.
Recently, cellular based system design is used as a foundation for smaller systems, such as wireless office systems (WOS). These wireless office systems may share the spectrum with the public cellular systems, also referred to as outdoor systems. Being smaller in scale than a public cellular system, a wireless office system utilizes less extensive processing systems. A WOS uses lower powered transceivers in radio heads for communicating in a localized area. The radio heads can also be considered as base stations. Cellular system frequency planning schemes are often not suitable for the smaller wireless office systems. Frequencies are not assigned to a transceiver or radio head. Instead frequencies are allocated by the WOS. The frequencies are a pooled resource common for all transceivers and base stations in the WOS. Thus, a mobile station operating in a WOS may retain the same frequency even after being handed off to a different transceiver in another base station. The mobile station need not re-tune.
Frequency planning of aWOS has to take into consideration several conflicting goals. These include that the system should not disturb the external public systems, the system should always have available operating frequencies and operation and maintenance should be simple.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention an adaptive frequency allocation system and method allocates the least interfered frequencies to be frequencies for use as pooled resources and prevents use of frequencies that potentially interfere with other systems.
Broadly, there is disclosed herein a method for automatically allocating which of available radio frequencies are to be used as a pooled resource for wireless communications in a wireless office system having a plurality of transceivers using the pooled frequencies. An interference level is determined for each radio frequency. Any radio frequency having an interference level above a select threshold is abandoned. Remaining usable radio frequencies having the lowest interference levels are selected as allocated frequencies to be used as pooled resources by any of the plurality of transceivers in the wireless office system.
It is a feature of the invention that received signal strength is measured for each of the radio frequencies. An interference level is determined only for idle radio frequencies.
It is another feature of the invention to filter the interference values. Particularly, filtered interference values are determined using low pass filters with short and long time constants. Radio frequencies are abandoned if they have filtered interference values above a select threshold. Radio frequencies having the lowest filtered interference values are selected as allocated frequencies.
It is another feature of the invention that the radio frequency is abandoned for a time period dependent on amount the interference value is above the select threshold. The time period is lengthened if the interference value remains above the select threshold after the channel frequency is abandoned.
It is still another feature of the invention that allocated frequencies are replaced with other usable radio frequencies only if the interference value for the allocated frequency is above an acceptable interference threshold.
It is still an additional feature of the invention that allocated frequencies are replaced only with other usable channel frequencies having lower interference values.
It is yet another feature of the invention that select ones of the allocated frequencies are designated as candidates for use as control channel frequencies. A serving control channel is periodically rotated among the candidates. The candidates are selected from the allocated frequencies having the lowest interference values. The serving control channel is rotated in a round robin manner.
It is disclosed in accordance with another aspect of the invention an automatic frequency allocation system in a wireless office system. The system includes measurement means for determining an interference level for each of a plurality of available radio frequencies. A plurality of transceivers communicate with mobile stations using any of allocated radio frequencies. A frequency allocator is operatively connected to the measurement means and the plurality of base stations and includes first logic means for abandoning any radio frequency having an interference level above a select threshold. Second logic means select remaining usable radio frequencies having the lowest interference levels as allocated frequencies to be used as a pooled resource by any of the plurality of transceivers.
More particularly, an adaptive frequency allocation system and method finds and maintains a set of frequencies for the wireless office system to use for traffic channels and control channels. These allocated frequencies are a pooled resource common for all the transceivers and base stations in the wireless office system. The method and system allocates the least interfered frequencies to the allocated frequencies, and prevents the wireless office system from using frequencies that potentially interfere with other wireless communication systems by removing such frequencies from the allocated frequencies. Received signal strength measurements are used to monitor idle frequencies. Bit error rate and receive signal strength measurements are used to monitor busy frequencies.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.